By Dieter Hardock, Dipl.-Ing., and Dave André, P.Eng., LEED GA
In Canada, it is widely stated buildings account for approximately 35 per cent of the total primary energy use, and roughly 30 per cent of the country’s total greenhouse gas (GHG) emissions. In the continuous quest to achieve better building energy performance, the design/construction community must develop and use innovative tactics to minimize operational energy use.
One significant aspect of energy loss involves conductive heat transfer through the building envelope, meaning the heat flow through solid elements due to temperature differences between interior space and exterior space. Common strategies to minimize this process by increasing the envelope’s overall R-value include:
- specifying high-performance glazing, such as improved double and triple-glazing with higher performance framing systems to reduce U-values;
- improving the façade insulation with increased R-values by using better and thicker insulation; and
- reducing the window-to-wall ratio (WWR), as windows are usually the thermally weakest areas of the wall.
These assemblies are largely responsible for the overall thermal performance of an exterior wall. Traditionally, not a lot of attention has been paid to the various thermal bridges that are integral to these larger envelopes because they were thought to represent a relatively small percentage of the overall energy loss.
As an example, one can consider the exposed concrete slab edges of a typical 1970s, lightly insulated, high-rise apartment building. The heat loss at the slab edge would be a relatively small percentage of the whole building’s losses. As the thermal performance of overall wall systems is improved, however, the heat loss through thermal bridges becomes a much greater percentage of total building energy loss, and thus more important to consider and control.
Although there are manufactured structural thermal breaks available for numerous connections, this article focuses on concrete balconies. Balconies remain a popular design feature for residential construction in Canada. Often formed by extending the concrete structural slab through the building envelope, these penetrations can represent a significant energy loss that can result in reduced thermal comfort and possible condensation.
Recent three-dimensional heat transfer analysis—published in American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 1365-RP, Thermal Performance of Building Envelope Details for Mid- and High-rise Buildings—indicates there can be as much as five times the amount of heat loss through an exposed concrete slab compared to an insulated one. The result is an increasing demand for, and interest in, thermal bridging solutions that will reduce these effects.
The popularity of off-the-shelf manufactured structural thermal breaks has steadily increased in Europe, thanks to this type of product’s performance, as well as the material and system testing being completed by those manufacturers. Now considered standard building practice across the Atlantic, these products have recently come to the Canadian market.
Thermal bridges are localized assemblies that penetrate insulated portions of the building envelope with thermally conductive materials. The associated heat loss results in a reduction of the indoor surface temperatures, which may create conditions for condensation and mould growth.
Generally speaking, there are many different structural elements that penetrate the building envelope and may form thermal bridges, such as balconies, canopies, slab edges, parapets, or corbels. These are common architectural features or essential structural elements in residential buildings as well as in hotels, schools, museums, or gyms.
In the case of uninsulated balcony slab connections, the interaction of the physical geometry of the balcony slab, the ‘cooling fin’ effect (i.e. increasing the exterior surface area leads to increased heat flow), and the material properties (i.e. the reinforced slab’s thermal conductivity) can result in significant heat loss.
Uninsulated balcony connections can be critical thermal bridges in a building envelope. Buildings relying on them have significant incentives for adoption of structural thermal break technology to improve thermal comfort, energy efficiency, and possibly indoor air quality (IAQ), by reducing mould growth potential.
Thermal break performance
Structural thermal breaks reduce heat flow between the inside to the outside, while also conserving structural integrity. With uninsulated balconies, for example, the reinforced concrete at the connection is replaced with an insulating material while continuous reinforcement bars are used to transfer moment and shear loads.
In some instances, these bars may be replaced by stainless steel where they penetrate the insulating material as this metal is much less thermally conductive than conventional reinforcing steel. The use of stainless steel not only reduces thermal conductivity, but also ensures longevity through its inherent corrosion resistance. Other materials are also used in some proprietary systems with the aim of lowering thermal conductivity, such as including concrete modules to transfer compression loads.
The combination of all these aspects means structural thermal breaks can average an equivalent thermal conductivity as low as 0.2 Watts per metre Kelvin (W/m·K), instead of typical values of 2.3 W/m·K for reinforced concrete at an untreated balcony connection. This reduces the thermal conductivity at the connection by up to 90 per cent, which significantly reduces the heat flow and also substantially improves the indoor surface temperature in the living area.
Typically, a range of structural thermal breaks are available from manufacturers, depending on the load requirements and deflection criteria. This selection allows for customizable solutions between structural and thermal performance to be found.